This invention relates to a method and apparatus for the measurement of the outside and inside diameters of a tubular object by ultrasonic techniques. More particularly, this invention relates to a method and apparatus including a plurality of fixed ultrasonic transducers in a fixed, phased array for dimensionally measuring an axially-mobile tubular object, such as a nuclear fuel rod, which includes calibration and temperature compensation. Still more particularly, this invention relates to a method and apparatus having a plurality of ultrasonic transducers in a fixed phased array about the circumference of an axial space through which a hollow tubular member passes, the phased array including a calibration and compensation wire in an ultrasonic circuit with the transducer, for calibrated and compensated measurement of the wall thickness, outside diameter, and inside diameter of the mobile tube.
In the prior art, a number of systems are known which relate to the measurement of the dimensions of an object by the emission of ultrasonic pulses and the reception of the echoes returned by the object, whereby the time separating transmission and the reception of the differing echoes enables a determination of the distance separating the various echo generating obstacles from the pulse-emitting sensor. Such techniques have been used to measure the dimensions of an object, such as a tubular object, for example, as shown in U.S. Pat. No. 3,929,006 (a system for measuring ultrasonically cable jacket thickness); the eccentricity of a tubular object, as in U.S. Pat. No. 3,426,437 and in U.S. Pat. No. 4,520,672; and thickness measurements by the use of ultrasonic probes for extruded plastic tubing as in U.S Pat. No. 4,740,146. Such systems are distinguishable from the use of an ultrasonic inspection device intended to measure and detect material flaws, such as in U.S. Pat. No. 3,121,324 and U.S. Pat. No. 4,487,072.
In a number of applications, knowledge of the dimensions and wall thickness of a tubular member are of interest. For example, in the nuclear industry, the integrity of the reactor control rod elements is directly dependent upon the strength of its cladding tube wall. In normal operating conditions, the reactor control rod elements are exposed to extremely complex thermodynamic and hydromechanic fluctuations. As a result of these conditions, rod vibration against its supporting structure in the reactor results in wearing of the tube wall in its cladding. The extent of this wear is directly related to rod integrity. Accordingly, monitoring and accurate measurements of these conditions are important steps in determining the status of control rod integrity. Thus, a number of systems and apparatus have been developed or proposed for such monitoring and measuring steps.
Based upon the measurements of the extent of wear of a control rod, it is possible to appropriately apply acceptance or rejection criteria for continuous operation of the control rod elements in the reactor.
In addition to the wear of a control rod element, some types of control rod elements because of the swelling of material in the hostile nuclear environment cause the cladding to bulge on the side of the tube. When bulges exceed a limited diameter, they can obstruct free rod movement through the structure in the reactor core. Thus, measurements of the eccentricity of the rod are thus important to determine when to remove the rod from the reactor in order to avoid its jamming in the supporting structure.
Presently-used techniques for measuring the control rod element profile are based generally on the use of eddy current principles. Two methods are used, one with a direct measurement of the rod surface lift-off effect on the array of stationary coils in the measuring fixture, while the second uses an indirect measurement of the lift-off effect through the mechnical fringes that ride the rod surface. These methods have limitations due to other electromagnetic property changes that can be encountered along the rod that affect the eddy current signal and significantly reduce measurement resolution. On the other hand, the magnitude of the signal amplitude responses limits the measuring range for what has been found impractical, especially when measuring the bulges. These two methods are widely used throughout the industry for production inspections on the control rod elements in operating nuclear power plants.
In addition to these production measuring techniques, a special test with a rotating ultrasonic probe has also been developed. This special test is used only for a limited rod location due to the very slow axial scanning speed. The results of these measurements, due to the complexity of the central rotational motion limitations, can be executed on the signal resolution. Thus, the method is primarily used to profile an outside surface of the rod as a follow up on an eddy current indication, or for inspecting an area where eddy current coverage is limited due to damages in the electromagnetic properties in the rod materials.
Accordingly, it is an overall objective of the invention to provide an ultrasonic measuring technique for directly measuring wall thickness with a plurality of focused ultrasonic sensors in an array mounted on a rod manipulator of the type conventionally used in the art. As is well known in the art, control rods which have operated within a nuclear reactor will suffer from radiation induced changes, as well as wear, cladding defects, swelling or ovality as noted above. In deciding whether to replace the rods or to continue their use, measurements of the type noted above are made to evaluate potential defects. A conventional control rod testing apparatus has an apparatus for supporting a plurality of elongated tubular control rods which are lowered through the test apparatus while located within the fuel storage pool. The testing apparatus may be a completely independent fixture, as noted in U.S. Pat. No. 4,670,211, assigned to the assignee of this invention, or may be a fixture secured to the top of a fuel bundle for the purpose of using the control rod guide tubes within the fuel bundle to aid in guiding the rods. The testing apparatus includes a guidance opening for all of the fingers of the control rod to be tested, but only selected portions will have a testing subassembly located within them.
It is thus an objective of this invention to provide a fixed ultrasonic testing mechanism, including a sensor and the physical holders constructed in a manner in which a conventional apparatus of the type described may readily be used to measure a moving tubular element.
It is another problem in the art in utilizing an ultrasonic test method and apparatus with a coupling liquid to compensate for changes of the sound velocity of the coupling liquid as a function of temperature. It is well known that when testing workpieces with ultrasonic energies the energy is cyclically transmitted from an electroacoustic transducer to the workpiece via a coupling liquid, which usually is water. The ultrasonic energy transmitted is reflected at the workpiece surface at an acoustic discontinuity within the workpiece and also at the rear surface of the workpiece. The transit time of the ultrasonic pulse while traversing the coupling liquid is an important parameter for determining the geometry of the workpiece under test. However, for accurate measurement, it is desirable that the acoustic velocity of the coupling liquid remain constant during the measuring period, or that an arrangement be provided for compensating the system for the influence of temperature. In U.S. Pat. No. 4,254,660, that problem is recognized and several solutions are discussed. The first includes regulating the temperature of the coupling liquid which, in this environment, is impractical. The second arrangement compensates for the sound velocity and is a mechanism of interest for this invention. Thus, it is desired to provide a compensation circuit in cooperation with the fixed phased array of ultrasonic transducers for compensating for temperature changes of the coupling liquid.
In addition, quality control requires that the instruments be periodically calibrated. Thus, it is another overall aim of this invention to provide a calibration circuit and mechanism in cooperation with the phased transducer array so that the circuit can be periodically calibrated to determine its accuracy and the functioning of any particular ultrasonic transducer.
Accordingly, it is an overall objective of this invention to provide a method and apparatus for ultrasonically measuring the outside diameter, inside diameter, and wall thickness of a tubular element in cooperation with a compensation/calibration system.
It is an additional overall object of this invention to provide an array of ultrasonic transducers circumferentially spaced about an axially-movable tube for directly measuring the diameter and wall thickness, while producing a profile of the outside diameter of the rod on a continuous basis.
It is an additional object of this invention to provide a fixed array of ultrasonic transducers spaced circumferentially about a path of travel for a hollow rod for directly measuring the wall thickness and outside diameter of the rod and arranged so that the transducers are structurally adapted for use in a rod tester of the conventional type used in the nuclear industry.
It is still another object of this invention to provide a simple yet accurate method of temperature compensation for temperature changes in the coupling fluid located between the transducer face and the rod surface.
It is an additional object of this invention to provide a wire which acts as a reflector surface for a transducer pulse acting as a reference because its location from the transducer face is fixed, whereupon the instrument may be calibrated and the readings compensated for changes in temperature of the coupling liquid, without requiring an additional transducer.
It is still another object of this invention to provide a reflector wire of the type described, in cooperation with each of a plurality of ultrasonic sensors positioned about the circumference of a path of travel of a nuclear control rod.
These and other objectives of the invention will become apparent from the detailed description of the drawings, taken in conjunction with the following written description of the invention.